Increased concentrations of the amino acid homocysteine in blood are recognized as a risk factor for thrombosis and atherosclerosis. Cysteine is a product formed from homocysteine, and increased concentrations of this amino acid may also serve as a risk factor for thrombosis and atherosclerosis. The mechanism of how these amino acids cause or serve as markers for these disorders is not known. Most of these amino acids are bound to albumin or other plasma proteins and mechanisms of exchange of these amino acids are not fully understood. Our studies are examining the distribution of homocysteine and cysteine among plasma proteins and mechanisms of exchange between free and protein-bound forms of these amino acids. Goals of these studies are: 1) To identify which measurements of plasma homocysteine, cysteine, or combinations of amino acid measurements serve as the best risk indicator, 2) To better understand the metabolic relationships of homocysteine and cysteine, 3) To determine whether these amino acids have direct effects on function of plasma proteins. A starting point for examining interactions with proteins is to study the binding and exchange of cysteine and homocysteine with different plasma proteins.